Non-Leaded Semiconductor Package and a Method to Assemble the Same

ABSTRACT

A method to assemble a non-leaded semiconductor package ( 1 ) comprises the following steps. A carrier tape ( 13 ) is attached to a metal foil ( 12 ). A plurality of leadframes ( 3 ) is formed in the metal foil ( 12 ), each leadframe ( 3 ) comprising a die pad ( 4 ) laterally surrounded by a plurality of contact leads ( 5 ). A semiconductor die ( 2 ), including an active surface with a plurality of die contact pads ( 7 ), is attached to each die attach pad ( 4 ) and electrically connected to the leadframe ( 3 ) by a plurality of bond wires ( 9 ) connecting the die contact pads ( 7 ) and the lead contact areas ( 6 ) of the contact leads ( 5 ). A plurality of leadframes ( 3 ), each including a wire bonded semiconductor die, are encapsulated with mold material ( 10 ). The carrier tape ( 13 ) is removed and the non-leaded semiconductor packages ( 1 ) separated.

The invention relates to a non-leaded semiconductor package and to amethod of assembling a non-leaded semiconductor package.

U.S. Pat. No. 6,498,099 discloses a method to produce a leadlesssemiconductor package by half etching one or both sides of the leadframestrip. After the wire bonding and molding processes, a further etchingprocess is performed to isolate and expose the contact pads.

This process comprises many processes steps and, in particular, manyetching steps. Etching is a slow and, therefore, expensive manufacturingprocess.

It is one object of the invention to provide a non-leaded semiconductorpackage and a simpler and more cost-effective method for producing thepackage.

This object of the invention is solved by the subject matter of theindependent claims. Further improvements arise from the subject matterof the dependent claims.

A non-leaded semiconductor package is assembled using a leadframe stripassembly. A method to produce the leadframe strip assembly according tothe invention comprises the following steps. Firstly, a metal foil isprovided and a carrier tape attached to one surface. A plurality ofleadframes is then formed in the metal foil, each leadframe comprising adie pad or die attach pad laterally surrounded by a plurality of contactleads. Each leadframe comprises the design of the desired semiconductorpackage. Preferably, the plurality of leadframes is formed by an etchingprocess which is, more preferably, performed from one side of the metalfoil. A plurality of isolated individual leadframes is formed.

By performing only one etching process the manufacturing processaccording to the invention is simplified. Performing the etching processfrom only one side of the metal foil leads to a simplification of theequipment required to form the leadframes and lower manufacturing costs.

Alternatively, a plurality of leadframes is formed in the metal strip bylaser cutting or stamping or any method known in the art. In thisembodiment of the invention the leadframes are attached to each other bythin metal joining strips and form a continuous leadframe strip. Theleadframe strip is then attached to the adhesive coating of a carriertape. The metal joining strips are then removed by, for example, etchingor by laser cutting to form a plurality of isolated leadframes attachedto the carrier tape. The etching process is performed from only one sideof the leadframe strip. This method has the advantage that the lasercutting or stamping process is relatively fast so that the manufacturingtime is reduced.

The leadframe strip assembly according to the invention thereforecomprises a metal foil attached to a carrier tape. The metal foilincludes a plurality of leadframes preferably arranged in a regulararray of columns and rows in the metal foil. Each leadframe comprises adie attach pad laterally surrounded by a plurality of contact leads. Thearrangement of the contact leads and the die attach pad relates to thedesired package design.

Preferably, the die pad and contact leads of each leadframe of theleadframe strip include anchorage features. Typically, the side walls ofthe die pad and inner side walls of the contact leads include protrudingportions formed by an approximately square cut-out section at the baseof the side walls. The protruding portion therefore extendsapproximately horizontally from the side walls of the die attach pad andinner side walls of the contact leads and typically has an approximatelysquare cross-section. The upper surface of the protruding portion lieson approximately the same plane as the upper surface of the die attachpad and contact leads. This protruding portion is the anchorage featurewhich advantageously improves the reliability of the package.

Preferably, the die pad and contact leads of each leadframe of theleadframe strip assembly are spatially isolated from each other and,preferably, each leadframe of the metal foil of the leadframe stripassembly is spatially isolated from its neighbour. The die pad andcontact leads of each leadframe are laterally isolated and are notattached to each other. Each leadframe is also laterally isolated and isnot attached to the neighbouring leadframes. The isolated leadframes aremechanically supported by the carrier tape. This arrangement of theleadframe strip assembly according to the invention is advantageous inthat the cutting and stripping processes used in conventional leadframemanufacturing processes are avoided. This manufacturing line istherefore simplified.

Additionally, by isolating the contact leads, a space is created betweenadjoining leadframes. Therefore, during the singulation process thecontact leads are not cut which is extremely advantageous.

Preferably, the carrier tape comprises a polyimide film with a siliconeadhesive coating. This carrier tape material has the advantage that ithas good heat resistance and, therefore, provides good mechanicalsupport to the leadframes through the manufacturing process and, inparticular, during the molding process. Also the adhesive can be cleanlyremoved from the bottom surface of the molded leadframe module or panelat the end of the manufacturing line just prior to the singulation ofthe individual semiconductor packages. This reduces the complex andcostly cleaning steps.

Preferably, the metal foil comprises copper or aluminium or one of theiralloys and more preferably comprises oxygen free high conductivity(OFHC) copper. These materials have good electrical conductivity, arerelatively inexpensive and can be easily processed.

The metal foil preferably comprises a thickness of approximately 1 mm toapproximately 0.01 mm or more preferably approximately 0.25 mm toapproximately 0.1 mm. The leadframe strip assembly according to theinvention is advantageous in that the thickness of the metal foil whichis used to form the plurality of leadframes can be thinner than thatused in conventional processes as the carrier tape provides mechanicalsupport. Also, in the method according to the invention the wholesurface area of the metal strip is not thinned by an etching process tocreate leadframes of the desired thickness. The materials cost andmanufacturing time is, therefore, reduced.

The leadframe strip may be partly or completely covered by anelectro-plated coating. Different parts, such as the die attach pad, thecontact leads and the contact areas of the contact leads, of a leadframemay be coated in different materials. The electro-plated coating maycomprise silver, nickel/palladium/gold or nickel/nickel phosphorous.

In the next stage of the process a semiconductor die, which includes anactive surface with a plurality of die contact pads and a passivesurface, is attached to each die attach pad of the leadframe stripassembly. Each die is electrically connected to the leadframe by aplurality of bond wires connecting the die contact pads and the leadcontact areas of the contact leads.

The plurality of dies attached to the leadframe strip assembly, contactleads, wire bonds and upper surface of the carrier tape are thenencapsulated with mold material forming a panel or molded leadframemodule. The mold material is typically a polymeric-based material.

A method to assemble a non-leaded semiconductor package according to theinvention comprises the following steps. The panel or molded leadframemodule formed by the leadframe strip assembly process is provided andthe carrier tape attached to the bottom surface is removed. Theindividual non-leaded semiconductor packages are singulated from thepanel by sawing.

The sawing process may be performed by a saw blade, water jet or laser.The leadframe strip assembly according to the invention provides a panelin which the packages are singulated by cutting through only the moldmaterial as the leadframe strip assembly includes isolated leadframes,die attach pads and contact leads. This greatly simplifies the sawingprocess.

If a saw blade is used to cut through two different materials, crackscan form between the two materials, burr formation is a problem and thewear rate of the saw blade is increased. Water jet technology isextremely slow and, therefore, may be impractical for high volumeproduction as it is too costly. If a laser beam is used to cut throughtwo types of material then the intensity must be adjusted which leads toa complicated process. These problems are avoided by the leadframe stripassembly according to the invention.

The leadframe strip assembly and the method of the invention isadvantageously used to produce very thin quad flat non-leaded (VQFN)packages and particularly for packages of the pancake design leadframewhich include a plurality of adjoining leadframes.

An embodiment of the invention will now be described by way of examplewith reference to the drawings.

FIG. 1 shows a non-leaded semiconductor package according to theinvention,

FIG. 2 shows a cross-sectional view of a section of a copper foil with acarrier tape according to the invention,

FIG. 3 shows a cross-sectional view of a section of the copper strip ofFIG. 2 after an etching process to form a plurality of isolatedleadframes, each having isolated contact leads,

FIG. 4 shows a cross-sectional view of a section of the leadframe stripassembly of FIG. 3 after a die attach and a wire-bonding process,

FIG. 5 shows a cross-sectional view of a section of the leadframe stripassembly of FIG. 4 after a molding process, and

FIG. 6 shows a cross-sectional view of a section of the leadframe stripassembly of FIG. 5 during a singulation process.

FIG. 1 shows a non-leaded semiconductor package 1 according to theinvention. The package 1 comprises a semiconductor die 2 and a leadframe3. The leadframe 3 comprises a die attach pad 4 in approximately itslateral centre which is laterally surrounded by a plurality of contactleads 5. The die attach pad 4 and contact leads 5 have essentially thesame thickness and lie on essentially the same lateral plane. The innerends of the contact leads 5 include lead contact areas 6. The side wallsof the die attach pad 4 and the inner side wall of the contact leads 5include a protruding portion 19 whose upper surface lies onapproximately the same plane as that of the upper surface of the dieattach pad or contact lead. The protruding portions 19 have anapproximately square cross-section. The leadframe 3 comprisesoxygen-free high conductivity copper.

The semiconductor die 2 includes an active surface with a plurality ofdie contact pads 7 and a passive surface. The passive surface of the die2 is attached approximately in the lateral centre of the die attach pad4 by die attach material 8. The die 2 is electrically connected to theleadframe 3 by a plurality of bond wires 9 which connect the die contactpads 7 and the lead contact areas 6.

The upper surface of the die 2, contact leads 5, bond wires 9 and spacebetween the die pad 4 and contact leads 5 is encapsulated with moldmaterial 10. The bottom surface 11 of the non-leaded package 1 comprisesmold material 10 and the bottom surfaces of the die attach pad 4 andcontact leads 5 on an essentially common plane. The outer side surfacesof the contact leads 5 are covered by a thin coating of the moldmaterial 10. The outer sides of the semiconductor package 1 areessentially vertical. The bottom surfaces of the contact leads 5 providethe external contact areas of the package 1.

FIG. 2 shows a copper foil 12 attached to a carrier tape 13 in the firststep of the method according to the invention to manufacture very thinquad flat non-leaded (VQFN) packages. The copper foil comprisesoxygen-free high conductivity copper and comprises a thickness ofbetween approximately 0.25 mm and approximately 0.1 mm. The copper strip12 is attached to the adhesive coating 17 on the upper surface of acarrier tape 13 which comprises a polyimide film substrate with a layerof silicone adhesive 17 on its upper surface.

FIG. 3 shows the next stage of the method according to the invention inwhich a plurality of leadframes 3 are formed in the copper foil 1. Theleadframes 3 formed by a selective etching process which takes place onone side, from the top as shown in FIG. 2, of the copper strip 12. Theupper surface of the carrier tape 13 acts as an etch stop.

The leadframes 3 are laterally arranged in a regular array of rows andcolumns in the copper foil 12. Each leadframe 3 includes a die attachpad 4 in the centre which is laterally surrounded by a plurality ofcontact leads 5. The lateral arrangement of the plurality of leadframesand the lateral arrangement of each individual leadframe cannot be seenin the cross-sectional views of the figures.

The contact leads 5 are separate from the die attach pad 4 and are notconnected to each other. Each leadframe 3 is laterally isolated from theneighbouring leadframes in the copper foil 12 and the bottom surface ofeach leadframe is attached to the adhesive coating 17 of the carriertape 13. The carrier tape 13 provides the mechanical support to theisolated leadframes 3, each including an isolated die attach pad 4 and aplurality of isolated contact leads 5 during the next stages of themanufacturing process up to just prior to the singulation process.

In the next step in the process, shown in FIG. 4, a semiconductor die 2is attached using die attach material 8 to the die pad 4 of eachleadframe 3 in the copper foil 12. The semiconductor die 2 includes anactive upper surface including a plurality of die contact pads 7. Theinner portion of each contact lead 5 of the leadframe 3 also includes acontact area 6. The electrical connection between the contact pads 7 ofthe semiconductor die 2 and the contact areas 6 of the contact leads 5of the leadframe 3 is formed by wire bonds 9.

FIG. 5 shows the molding process of the method according to theinvention. The plurality of leadframes 3 is encapsulated by moldmaterial 10 to form a molded leadframe module or panel 14. The die 2,die attach pad 4, contact leads 5, wire bonds 9 and areas between thecontact leads 5, die attach pads 4 and the upper surface of the carriertape 13 of each of a plurality of leadframes 3 are encapsulated by asingle mass of mold material 10. The upper surface of the carrier tape13 acts as the bottom surface of the mold. The mold material is thengiven an appropriate curing treatment.

FIG. 6 shows the final step of the process in which the carrier tape 13is removed from the molded leadframe module 14. The bottom surface 16 ofthe molded leadframe module 14 comprises mold material 10 and isolatedareas comprising the bottom metal surfaces of the die attach pads 4 andisolated contact leads 5 on an essentially common surface. Theindividual non-leaded packages 1 are singulated from the moldedleadframe module 14 by sawing through the mold material 10 between theindividual leadframes 3 as indicated by the dotted lines 15 and sawblade 18.

Contacting means, such as solder balls, are then attached to the bottomsurface of the contact leads 5 and provide the external contacts fromthe non-leaded package 1 to, for example, a printed circuit board.Alternatively, the solder balls may be attached to the contact leads 5while the package forms part of the molded leadframe module. Thepackages 1 are then tested, packages and then mounted on, for example, aprinted circuit board.

REFERENCE NUMBERS

-   1 non-leaded semiconductor package-   2 semiconductor die-   3 leadframe-   4 die attach pad-   5 contact lead-   6 contact lead contact area-   7 die contact pad-   8 die attach material-   9 wire bond-   10 mold material-   11 bottom surface of package-   12 non-leaded package-   13 carrier tape-   14 molded leadframe module-   15 singulation lines-   16 bottom surface of leadframe module-   17 adhesive coating-   18 saw blade-   19 protruding portion

1.-13. (canceled)
 14. A method to assemble a leadframe strip assemblycomprising: providing a metal foil; attaching a carrier tape to themetal foil; and forming a plurality of leadframes in the metal foil,each leadframe comprising a die pad laterally surrounded by a pluralityof contact leads.
 15. The method to assemble a leadframe strip assemblyaccording to claim 14, comprising forming the plurality of leadframes byan etching process.
 16. The method to assemble a leadframe stripassembly according to claim 14, comprising performing the etchingprocess from one side of the metal foil forming a plurality of isolatedleadframes.
 17. A leadframe strip assembly comprising: a carrier tapeincluding a metal foil attached thereon; and a plurality of leadframesformed in the metal foil, each leadframe comprising a die pad laterallysurrounded by a plurality of contact leads in the metal foil.
 18. Theleadframe strip assembly according to claim 17, comprising wherein thedie pad and contact leads of each leadframe of the metal foil arespatially isolated from each other.
 19. The leadframe strip assemblyaccording to claim 17, comprising spatially isolating each leadframe ofthe metal foil from its neighbor.
 20. The leadframe strip assemblyaccording to claim 17, wherein the carrier tape comprises a polyimidefilm with a silicone adhesive coating and the metal foil comprises OFHCCu.
 21. The leadframe strip assembly according to claim 17, wherein themetal foil comprises a thickness of approximately 1 mm to approximately0.01 mm or approximately 0.25 mm to approximately 0.1 mm.
 22. Theleadframe strip assembly according to claim 17, wherein the leadframestrip assembly further comprises a plurality of semiconductor die, eachincluding an active surface with a plurality of die contact pads and apassive surface, attached to the die attach pads and electricallyconnected to the leadframe by a plurality of bond wires connecting thedie contact pads and the lead contact areas of the contact leads. 23.The panel comprising a section of the leadframe strip assembly accordingto claim 22, comprising encapsulating the plurality of dies, contactleads, wire bonds and upper surface of the carrier tape with moldmaterial.
 24. A method to assemble a non-leaded semiconductor packagecomprising: providing a panel according to claim 10; removing thecarrier tape; and singulating the non-leaded semiconductor packages. 25.A non-leaded semiconductor package comprising: a leadframe comprising adie attach pad approximately in its lateral centre, laterally surroundedby a plurality of contact leads each having a contact area;semiconductor die including an active surface with a plurality of diecontact pads and a passive surface, attached to the die attach padelectrically connected to the leadframe by a plurality of bond wiresconnecting the die contact pads and the lead contact areas of thecontact leads; the upper surface of the die, contact leads, bond wiresand space between the die pad and contact leads being encapsulated withmold material; and the bottom surface of the non-leaded packagecomprising mold material and the bottom surface of the die attach padand contact leads on an essentially common plane.
 26. The non-leadedsemiconductor package according to claim 25, wherein the leadframecomprises a thickness of approximately 1 mm to approximately 0.01 mm.27. The non-leaded semiconductor package according to claim 25, whereinthe leadframe comprises a thickness of approximately 0.25 mm toapproximately 0.1 mm.
 28. A non-leaded semiconductor package having aleadframe strip assembly comprising: a semiconductor die; a carrier tapeincluding a metal foil attached thereon; and a plurality of leadframesformed in the metal foil, each leadframe comprising a die pad laterallysurrounded by a plurality of contact leads in the metal foil.
 29. Thenon-leaded semiconductor package according to claim 27, comprisingwherein the die pad and contact leads of each leadframe of the metalfoil are spatially isolated from each other.
 30. The non-leadedsemiconductor package according to claim 28, comprising spatiallyisolating each leadframe of the metal foil from its neighbor.
 31. Thenon-leaded semiconductor package according to claim 29, wherein thecarrier tape comprises a polyimide film with a silicone adhesive coatingand the metal foil comprises OFHC Cu.
 32. The non-leaded semiconductorpackage according to claim 30, wherein the metal foil comprises athickness of approximately 1 mm to approximately 0.01 mm orapproximately 0.25 mm to approximately 0.1 mm.
 33. The non-leadedsemiconductor package 31, wherein the leadframe strip assembly furthercomprises a plurality of semiconductor die, each including an activesurface with a plurality of die contact pads and a passive surface,attached to the die attach pads and electrically connected to theleadframe by a plurality of bond wires connecting the die contact padsand the lead contact areas of the contact leads.
 34. A leadframe stripassembly comprising: means for providing a carrier tape including ametal foil attached thereon; and means for providing a plurality ofleadframes formed in the metal foil, each leadframe means comprising adie pad laterally surrounded by a plurality of contact leads in themetal foil.